1. Field of the Invention
This invention relates in general to earth-boring bits and in particular to earth boring bits of the type having rotatable cutters with earth disintegrating teeth.
2. Background Information
Earth-boring bits fall generally into two categories: (1) drag bits with a variety of forms including those with synthetic or natural diamond used for cutting elements; (2) those with rotatable cutters having earth disintegrating teeth formed of steel and other suitable metals, such as sintered tungsten carbide.
The rotatable cone type bits have generally two or three cones. The three-cone bit has enjoyed the greater commercial success for a number of reasons, including the fact that they "run smooth". Two-cone bits tend to run rougher, a condition that generates vibration in the bit and drill string that impedes drilling and tends to be detrimental to the drilling rig and equipment. This is because two-cone bits generally contact the sidewall or gage of the borehole at two points, one for each of the cutters. If the two cutters are symmetrical, i.e. 180.degree. apart, with similar offset relative to the axis of rotation of the bit, undesirable dynamics such as backward whirl and off-center rotation can occur. The problem is also present in three-cone bits, but is reduced in magnitude due to the three contact points between the three cutters and the sidewall of the borehole.
Commonly assigned U.S. Pat. No. Re. 34,526, Feb. 1, 1994, to Pessier discloses a two-cone bit in which one of the cutters is skewed such that the cutters are non-opposite and the symmetry referenced above and associated rough running are avoided.
It is advantageous to utilize two-cone bits, if they can be made to run smooth, in some types of earth formations. For example, softer formations can be effectively drilled with two-cone bits, which can have long, more aggressive teeth or cutting elements and permit larger return flow area for the removal of large volumes of cuttings. Additionally, any improvements to the running characteristics of rotatable cutter bits result in longer bit life.